The strength of the Barcelona's Eixample comes from its urban plan, a visionary project designed by Ildefons Cerdà in 1859. The original project proposed blocks with a central open space built on two or three sides, allowing the introduction of variation through the urban fabric. However, to increase density the courtyards were privatized thereby reducing the public realm to the streets. The process was nevertheless reversed in 1986 by a new bylaw promoting the recuperation of the courtyards for public space. This project reasserts Cerdà's agenda for a diverse public realm forming a fluid network through streets and reclaimed courtyards and creating unique localities along promenades within a repetitive system. To solve the relation between users and programs, the different uses can coexist by a set of levels, taking up the existing fragmentation of buildings' terraces. Housing is extended by communal gardens, preserving privacy, while cafés and retail expand their activities on the interior and children and the elderly find social activities. The design of these itineraries is solved through parametric optimization, using a gaming pathfinder algorithm, with emphasis on the privacy condition of each block. The engine uses agents (virtual people) to determine the appropriate location and permeability of the threshold between street and courtyard. It is able to manage systematically small, human-scale interventions to (re) introduce a walking dimension into Cerdà's urban fabric.